From DE 10 2005 002 337 A1 a transmission device designed as an 8-gear multi-step transmission is known, which comprises frictional shifting elements such as disk clutches and disk brakes. When a shift demand is made for a gearshift in the transmission device, at least one frictional shifting element which is engaged in order to obtain the actual gear currently engaged in the transmission device has to be disengaged out of the force flow of the transmission device, whereas at least one other frictional shifting element, which is disengaged from the force flow of the transmission device while the actual gear is currently engaged in the transmission device, has to be engaged in the force flow of the transmission device in order to obtain the required gear.
During this, with increasing shifting time the torque transmitted by the frictional shifting element engaged in the force flow in order to obtain the current actual gear of the transmission device is transferred to a greater or lesser extent to the frictional shifting element that has to be engaged in the force flow of the transmission device in order to obtain the required target gear, while the torque transmitted by the shifting element to be disengaged decreases.
Disadvantageously, frictional shifting elements in the open operating condition give rise to drag torques which impair the overall efficiency of an automatic transmission to an undesired extent.
For that reason transmission devices such as that known from DE 10 2008 000 429 A1 are to an increasing extent designed, besides frictional shifting elements, also with interlocking shifting elements in the area of which no drag torques that impair the overall efficiency of a transmission device occur.
In that case, however, it must be borne in mind that interlocking shifting elements can only be shifted when close to their synchronous point from an open operating condition, in which no torque can be transmitted by the interlocking shifting element, to their closed operating condition, in which the full torque applied can be transmitted by the interlocking shifting element. In addition, interlocking shifting elements engaged in the force flow of a transmission device can only be disengaged from the force flow by means of low shifting forces when they are close to their load-free operating condition. In contrast to frictional shifting elements, both for the synchronization of interlocking shifting elements and also to change interlocking shifting elements to their load-free condition, additional constructional devices are needed in order to enable shifts in transmission devices that involve at least one interlocking shifting element to be carried out within desirable shifting times. In addition, with the last-described transmission devices made with interlocking shifting elements as well, when the previously initiated gearshift from the current gear toward the target gear has to be interrupted, the possibility also exists that the interlocking shifting element involved in the shift cannot be changed to its operating condition required for obtaining the currently engaged gear, i.e. to its open or to its closed operating condition, for example because of the torque applied.
In methods known from practice for operating such transmission devices, if it becomes necessary to interrupt the shift currently in progress it is usual to change back into the currently engaged gear. However, in the case of gearshifts during which, to obtain the required target gear, interlocking shifting elements have to be opened or closed, this is not possible at any time during the shift since when closing, due to the torque build-up interlocking shifting elements can no longer be opened from a certain time-point onward or, during an opening phase after a complete release of the interlock in the area of the interlocking shifting element, they can no longer be changed to their closed operating condition without an additional synchronization phase. This means that after the passing of the time-points, hereinafter called the points-of-no-return and which are characteristic for various gearshift types, a shift that has already begun in the direction toward the required target gear can no longer be interrupted without changing the transmission device to an undefined operating condition.